This application claims priority to an application entitled, xe2x80x9cImage Tracking Device and Method for Transverse Measurement of Optical Fiber,xe2x80x9d filed in the Korean Industrial Property Office on Aug. 22, 2000 and there duly assigned Ser. No. 2000-48506.
1. Field of the Invention
The present invention relates generally to an image tracking device in an optical communication system, and in particular, to a device and method for measuring the transverse characteristics, including the refractive index or residual stress, of an optical fiber or a fiber preform.
2. Description of the Related Art
Hereinafter, the term, xe2x80x9ctransverse section,xe2x80x9d relating to an optical fiber (or preform) will refer to the section perpendicular to xe2x80x9cthe lengthwise sectionxe2x80x9d of the optical fiber (or preform), whereas the term, xe2x80x9clongitudinal sectionxe2x80x9d will refer to the section that is parallel to xe2x80x9cthe length directionxe2x80x9d of the optical fiber (or preform).
Various types of image-tracking techniques for tracking the transverse and longitudinal images of the optical fiber (or preform) are available. The transverse measuring device is typically utilized to detect the residual stress and refractive index of an optical fiber (or fiber preform). Despite having a low-image resolution, the transverse measuring device allows a non-destructive testing environment compared to other measurement techniques that are available in the industries related to fiber and fiber fabrication device. Moreover, unlike the longitudinal measuring technique, a polarization distribution effect can be measured accurately using the transverse measuring technique. Therefore, the transverse measurement is more preferred for measuring the characteristics of an optical fiber (or preform).
FIG. 1 illustrates a conventional measuring device for enabling the longitudinal measurement of an optical fiber. For the purpose of illustration, an optical fiber 14 is arranged along the z-axis direction, and the longitudinal section 15 of the optical fiber 14 is aligned in the x-y plane. Parallel light 13, emitted from a light source 11, is focused on the longitudinal section 15 of the optical fiber 14 by a first lens 12. Some portion of the light 13 incident on the transverse section 15 is transmitted into the optical fiber 14, while the other portion of the light 13 is reflected. The reflected light 13 from the fiber 14 is coupled into a second convex lens 16 in backward direction and thereafter determined by an optical detector 17 as light power. Accordingly, the optical detector 17 measures the power of the reflected light received thereon, and the measured power is used to obtain information about the refractive index of the parallel light 13 at the beam spot on the transverse section 15 of the fiber 14. Hence, by implementing this type of parallel light measuring device, the refractive index distribution of the transverse section 15 of the fiber 14 can be derived using the power of the light detected at the detector 17.
FIG. 2 illustrates another conventional measuring device for detecting the transverse characteristics of an optical fiber. As shown in FIG. 2, the optical fiber 23 is arranged along the z-axis direction, and the longitudinal section 24 of the optical fiber 23 is aligned along the x-z plane,. Light 22 emitted from a light source 21 passes through the fiber surface and the longitudinal section 24 of the optical fiber 23 and eventually arrives at the image sensor 25. Accordingly, the image of the longitudinal section 24 can be measured and can determine whether the optical fiber 23 contains beam deflecting sections, which result from different refractive indexes by dopants. To achieve this, the conventional measuring device also includes an image sensor (i.e., CCD) 25 to measure the intensity distribution of light passing through the fiber component 24. Hence, the image of the intended longitudinal section of an optical fiber, including the center of the optical fiber as well as the sectional profiles of an optical fiber, is detected.
However, there are some drawbacks with the above-identified conventional systems, which rely on the diffraction of an optical fiber image. Typically, the transverse image of an optical fiber is detected not by projecting light onto a sample but by transversely radiating the optical fiber with light and then detecting refracted light therefrom. When light is projected onto the outer circumferential surface of the optical fiber, the cylindrical core structure acts as a lens. That is, an optical fiber composed of a core and a cladding with different refractive indices and with a symmetrical cylinder shape has equivalent function as a lens. Thus, the light focusing effect and the light diffraction effect are generated when light passes before and behind the center of the core, respectively. Currently, there is no way to numerically analyze these focusing and diffraction effects caused by the fiber in the conventional image sensor. Therefore, there is a need for a new image-tracking method that is capable of detecting the focusing and diffraction effects caused by the fiber core member.
The present invention relates to an image-tracking device that can minimize measurement errors caused by the fiber core member in the transverse measurement method.
Accordingly, an image-tracking device that is capable of detecting the transverse characteristics of an optical fiber is provided and includes a linear object; a light source for emitting light onto the light object; a first convex lens for projecting the light received via the linear object onto the outer circumferential surface of the optical fiber and forming a primary image of the linear object penetrating the optical fiber; a second convex lens for converging the light received via the optical fiber and forming a secondary image of the linear object; an image sensor for detecting the secondary image; and, a controller for calculating the distance between the primary image and the center of the optical fiber based on the distortion degree on the detected secondary image.
The present invention provides a method for measuring the transverse characteristics of an optical fiber, a linear object is arranged to be inclined at an angle other than 90xc2x0 with respect to the transverse direction of the optical fiber. Light is projected onto the linear object and the light that passes through the linear object is focused thereafter. A primary image of the linear object is generated to be within the optical fiber. A secondary image of the linear object is generated by focusing the light that has passed through the optical fiber with a lens 70. As a consequence, the secondary image is detected and a differential curve is derived from the detected secondary image. Finally, the distance between the primary image and the center of the optical fiber is calculated according to the length of a distortion region and distortion peaks on the differential curve.